The present invention relates to a naphthoquinone derivative having excellent electric charge transferability, and an electrophotosensitive material containing the naphthoquinone derivative, which is used in image forming apparatuses such as antistatic copying machine, facsimile, laser beam printer and the like.
In the image forming apparatuses, so-called organic photosensitive materials comprising an electric charge generating material for generating electric charges by irradiation with light, an electric charge transferring material for transferring generated electric charges, and a binder resin constituting layer(s) in which these substances are dispersed have widely been used. In general, the organic photosensitive materials are classifies roughly into a photosensitive material having a single-layer type photosensitive layer wherein an electric charge generating material and an electric charge transferring material are contained in the same layer, and a photosensitive material having a multi-layer type photosensitive layer comprising an electric charge generating layer containing an electric charge generating material and an electron transferring layer containing an electron transferring material, which are mutually laminated. In the multi-layer type photosensitive layer, the electric charge transferring layer having a larger film thickness than that of the electric charge generating layer is generally provided on an outermost layer of the photosensitive material in view of the mechanical strength.
The electric charge transferring material used in these photosensitive materials includes those having hole transferability and those having electron transferability. Among electric charge transferring materials known at present, almost all of those having high carrier mobility, capable of imparting practical sensitivity to the photosensitive material, are those having hole transferability. Therefore, organic photosensitive materials, which are put into practice at present, are negative charging type photosensitive materials in case of the multi-layer type one wherein the electric charge transferring layer is provided on the outermost layer of the photosensitive material.
However, the negative charging type organic photosensitive material has problems such as influence of ozone on the environment and deterioration of the photosensitive material itself because it must be charged by negative corona discharge that causes evolution of a large amount of ozone.
To solve the above problems, it has been studied to use an electron transferring material as the electric charge transferring material. For example, Japanese Unexamined Patent Publication (Kokai) No. 1-206349 suggests that a compound having a diphenoquinone or benzoquinone structure is used as the electron transferring material.
However, since the compound having a diphenoquinone or benzoquinone structure generally has poor matching with the electric charge generating material, injection of electrons from the electric charge generating material into the electron transferring material is insufficient. Such an electron transferring material is not uniformly dispersed in a photosensitive layer because of its poor compatibility with a binder resin, so that a hopping distance of electrons becomes longer and electrons do not easily move in a low electric field.
Accordingly, a conventional photosensitive material containing an electron transferring material has a problem that the residual potential increases and the sensitivity becomes poor, as is apparent from electrical characteristics test described in the Examples described hereinafter.
The single-layer type photosensitive material has such an advantage that one photosensitive material can be used in any of positive charging type and negative charging type by using the electron transferring material in combination with the hole transferring material. However, when using a diphenoquinone derivative as the electron transferring material, there arises a problem that a charge transfer complex is formed by an interaction between the electron transferring material and hole transferring material, thereby to inhibit transfer of electrons and holes.
To improve the above problems , for example, Japanese Unexamined Patent Publication (Kokai) Nos. 6-110227 and 9-151157 suggest that a monomer of naphthoquinones is used as the electron transferring material.
However, the electric charge mobility is not sufficient in the monomer of the naphthoquinones. Therefore, the residual potential and photosensitivity were not satisfactory even when using this compound in the photosensitive material.
Thus, an object of the present invention is to solve the above technical problems, thereby to provide a novel compound suited for use as an electron transferring in an electrophotosensitive material.
Another object of the present invention is to provide an electrophotosensitive material whose sensitivity is improved as compared with the prior art.
The present inventors have intensively studied to attain the above objects and found a new fact that a naphthoquinine derivative represented by the general formula (1): 
wherein R1 and R2 are the same or different and each represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, has higher electron transferability than that of a conventional electron transferring material such as compound having a diphenoquinone structure or benzoquinone structure, and is superior in compatibility with a binder resin, thus completing the present invention.
Accordingly, the present invention relates to:
(i) A naphthoquinone derivative represented by the general formula (1).
(ii) An electrophotosensitive material comprising a conductive substrate and a photosensitive layer provided on the conductive substrate, wherein the photosensitive layer contains the naphthoquinone derivative represented by the general formula (1) of the item (i).
(iii) The electrophotosensitive material according to the item (ii), wherein an electron acceptor is contained in the photosensitive layer.
(iv) The electrophotosensitive material according to the item (ii), wherein the photosensitive layer is a single layer.
(v) The electrophotosensitive material according to the item (ii), wherein the photosensitive layer contains a phthalocyanine.
(vi) The electrophotosensitive material according to the item (ii), which is a positive charge type electrophotosensitive material.
(vii) The electrophotosensitive material according to the item (ii), which has an electric charge generating layer and an electric charge transferring layer on the upper side of the conductive substrate, wherein the electric charge generating layer is located inner side to the electric charge transferring layer.
Such a naphthoquinone derivative (1) is superior in electron acceptability, and is uniformly dispersed in a photosensitive layer because of its good compatibility with a binder resin. Injection of electrons from the electric charge material is smoothly conducted because of excellent matching with the electric charge material. Accordingly, the naphthoquinone derivative (1) exhibits excellent electron transferability even in a low electric field, and is suited for use as the electron transferring material in the electrophotosensitive material.
Furthermore, since the naphthoquinone derivative (1) does not form a charge transfer complex, together with the hole transferring material, it can be particularly suited for use in a single-layer photosensitive layer using the electron transferring material in combination with the hole transferring material.
The electrophotosensitive material of the present invention comprises a conductive substrate and a photosensitive layer provided on the conductive substrate, wherein the photosensitive layer contains the naphthoquinone derivative represented by the general formula (1).
Such an electrophotosensitive material has lower residual potential than that of a conventional electrophotosensitive material containing an electron transferring material and high sensitivity because a naphthoquinone derivative (1) having excellent characteristics described above is contained in a photosensitive layer.
That is, the photosensitive layer containing the naphthoquinone derivative (1) is superior in electron transferability in a low electric field and causes less rebinding of electrons and holes therein and, furthermore, an apparent electric charge generating rate approaches an actual value, thus improving the sensitivity of the photosensitive material having such a photosensitive layer. The residual potential of the photosensitive material is also lowered, thereby to improve the stability and durability on repeated exposure.
Since the naphthoquinone derivative (1) does not form the charge transfer complex, together with the hole transferring material as described above, a photosensitive material having higher sensitivity can be obtained when it is used in a single-layer type photosensitive material containing an electron transferring material and a hole transferring material in the same photosensitive layer.
The electron transferability is further improved when an electron acceptor is contained in the photosensitive layer, so that a photosensitive material having higher sensitivity can be obtained.